The present invention relates to an A/D converter for converting analog signals to digital signals and, more particularly, to a technique which is effective when used in an oversampling type A/D converter contained in a communication LSI for processing speech band signals, such as a MODEM LSI, an echo canceller LSI or CODEC LSI.
The oversampling type A/D converter relaxes the accuracy of an analog element formed in the A/D converter, to realize the desired conversion accuracy by using a sufficiently high sampling frequency for the Nyquist frequency (i.e., the double frequency of a signal frequency bandwidth). This A/D converter is classified into several types depending upon the combination of integration circuits contained in the main and feedback paths of signals, as disclosed in ICASSP 85, Proc. Technical Papers, 36.7.1 to 36.7.4 (1985), pp. 1400 to 1403.
Of these, the delta-sigma type A/D converter has no integration circuit in its feedback path to produce one bit/sample digital output on principle. In this type, the feedback signal from the D/A converter constructed takes positive or negative reference voltages equal to the maximum coding amplitude. If the voltage change of an analog input signal between two consecutive samples increases, there arises a phenomenon called the "gradient overload" that the follow-up response becomes the worse to deteriorate the S/N ratio for the larger number of conversion bits of the D/A converter. Since, however, the aforementioned D/A converter has one conversion bit, it has the frequency of the analog input signal free from that gradient overload, i.e., the maximum gradient overload frequency.
On the contrary, however, the quantization noise per one sample is high so that a higher sampling frequency has to be used for conversion characteristics of high accuracy, i.e., for a high S/N ratio. In the analog integration circuit formed with the aforementioned delta-sigma type A/D converter, therefore, the maximum signal amplitude has to be integrated at a high speed and in a high accuracy to impose strict characteristics upon an analog element such as an arithmetic amplifier.
Thus, it has been proposed to improve the frequency characteristics of the quantization noise thereby to suppress the increase in the sampling frequency by cascading a plurality of stages of the delta-sigma type A/D converters, as disclosed in ISSCC 86, Dig. Technical Papers, THPM 14.6 (1986), pp. 182 to 183, and ISSCC 87, Dig. Technical Papers, WAM 3.5 (1987) pp. 48 and 49.